Don Roberto, you also asked for logistics:
I doubt that Sollozzo would try to bluff Vito by claiming that the Tattaglias would guarantee his investment if they hadn't agreed--it'd be too easy for Tom or Vito to uncover. Then the deal would not only die, but probably Sollozzo, too.

I imagine that if Vito had gone for the deal, Tom would have paid a visit to his counterpart in Tattaglia's organization to confirm that if the deal collapsed, or Sollozzo was arrested, imprisoned, deported or died before Vito started seeing money, the Tattaglias would remit Vito's million. This could have made for a very complex negotiation, with many subplots if GF had gone that way.